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Paleothermal data from the Cretaceous through Miocene Shimanto Belt reveal important variations in thermal history, both along and across strike within the ancient accretionary prism. Two types of variations are considered: those due to different tectonic settings (e.g., tectonic mélange versus coherent accreted rocks versus slope-basin deposits); and those due to different times of accretion. Within the Cretaceous subbelt on Shikoku, shale samples from mélange and coherent rocks show the same level of illite crystallinity, but samples from a probable slope-basin deposit are significantly less crystalline suggesting that the slope-basin strata experienced less heating than underlying accreted rocks. In contrast, within the Tertiary subbelt, mélange shales, coherent accreted strata, and slope-basin deposits all overlap with respect to illite crystallinity and vitrinite reflectance values; moreover, the grade tends to be higher than in the Cretaceous subbelt. This difference in grade may merely reflect different levels of exposure, but is more likely related to the fact that both the Eocene-Oligocene and Oligocene-Miocene subbelts experienced relatively high paleogeothermal gradients, apparently resulting from separate ridge subduction events in the late Eocene/early Oligocene and middle Miocene (DiTullio and others, this volume; Hibbard and others, this volume).

In addition to overall differences in grade between the Cretaceous and Tertiary subbelts, marked variations in metamorphism and deformation style in the Shimanto Belt are apparent along strike between the Shikoku and the neighboring Kii Peninsula and Kyushu. In Kyushu, all the subbelts of the Shimanto Belt are characterized by low dips. In the relatively highly metamorphosed Cretaceous subbelt and in the Kitagawa Group of the Eocene-Oligocene subbelt, this may reflect deformation in an underplating regime. However, in the Eocene to lower Oligocene Hyuga Group, as well as probably originally in the Paleogene Otanashigawa Group and the Oligocene-Miocene Muro Group of Kii Peninsula, moderate- to low-dipping strata are associated with only diagenetic grades of thermal alteration. Although the Hyuga and Muro Groups may represent slope-basin deposits, clearly accreted Eocene rocks such as the Kitagawa and Otanashigawa Groups have experienced markedly less landward rotation than equivalently thermally altered accreted rocks in Shikoku. This difference in landward rotation may be related to landward-vergent structures observed in the Oligocene-Miocene subbelt. Landward-vergent structures may have resulted from a seaward-dipping backstop caused by the high paleogeothermal gradient resulting from subduction of the Shikoku basin spreading ridge beneath the Muroto Peninsula in the middle Miocene (Byrne and Hibbard, 1987). However formed, the landward-vergent structures probably contributed to the enhanced steepening of strata in the Shimanto Belt in Shikoku with respect to other areas along strike.

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